The objective of this proposal is to establish a Statistical and Data Coordinating Center for the multicenter pacemaker Mode Selection Trial (MOST) in patients with sick sinus syndrome. The study is a two-arm, single blind, randomized clinical trial that will enroll 2,000 patients who require permanent pacemaker implantation. Qualifying patients will receive a dual chamber pacemaker, which prior to implantation, will be noninvasively programmed by random assignment to either single chamber ventricular pacing (VVIR) or dual chamber, rate-modulated pacing (DDDR). Following discharge, all patients will be followed via clinic visits at 1, 3, and 6 months, and at 6-month intervals thereafter. Patients will be recruited into the trial over a period of 3 years, with a subsequent minimum follow-up period of 1.5 years. The primary endpoint of the trial is a composite outcome consisting of all-cause mortality or nonfatal stroke. Important secondary endpoints include (1) quality of life and cost effectiveness, (2) total mortality, (3) cardiovascular mortality, (4) death, stroke, or congestive heart failure requiring hospitalization, (5) atrial fibrillation, (6) heart failure score, and (7) pacemaker syndrome. In collaboration with the Clinical Coordinating Center and the Quality of Life and Cost Center, the Data Coordinating Center will perform the following major functions: (1) participate in all phases of planning and study design; (2) coordinate the preparation of data collection forms and procedures; (3) prepare a manual of operations; (4) provide training and guidance in data collection procedures; (5) coordinate the randomization of patients; (6) efficiently organize the flow and management of all patient data; (7) establish and adhere to high standards of quality control for data management; (8) perform on-site monitoring of completed data forms; (9) prepare weekly status reports for the Clinical Coordinating Center and Quality of Life and Cost Center; (10) prepare regular reports for study committees; (11) coordinate and perform appropriate statistical analyses of study data; and (12) participate in the preparation of study publications. Noteworthy features of this proposal include: a detailed assessment of sample size requirements; telephone randomization of patients; double data entry; complete follow-up of all patients; on-site audits of data by trained nurse monitors; use of economical and efficient computer hardware and software; state-of-the-art methods of data analysis; and an experienced team of investigators. Through the services it provides, which extend across all aspects of study design, data collection, data management, and data analysis, the Data Coordinating Center will be a vital resource in the execution of this clinical trial. The results of this study are anticipated to provide important new clinical information and to greatly enhance the appropriateness of pacemaker therapy in patients with sick sinus syndrome.